fix(status): serve menubar-json status from a disk-persisted snapshot to eliminate per-poll re-parse latency

[dgabehar]

Summary

codeburn status --format menubar-json took 25-90+ seconds per call
regardless of cache state, causing the polling CodeBurnMenubar.app to
stall/stop refreshing. Root cause (confirmed via live profiling): the
menubar spawns a fresh CLI process per poll, so every call re-parses the
full session-cache blob and re-runs the full aggregation pipeline with
zero cross-process reuse — none of the existing in-process caches
(parser.ts's TTL/burst maps, session-cache.ts's cacheMemo) ever survive
between polls.

This adds a disk-persisted status snapshot keyed by a cheap corpus
fingerprint (stat-only pass over discovered sources — dev/ino/mtime/size,
no session-cache.json parse, no transcript content read), with a
settle-window debounce (CODEBURN_STATUS_SNAPSHOT_SETTLE_MS, default
2000ms, capped 60000ms) so rapid-fire source writes coalesce into one
recompute instead of one per poll tick.

Related

#971 covers the same general problem class (expensive redundant
per-invocation recompute) from a different angle — a resident
codeburn serve daemon sometimes being bypassed on cold start. This PR
is orthogonal to that: the menubar's status --format menubar-json path
has no resident-process story at all today (a fresh CLI process every
poll, always), so the fix here is a disk-persisted snapshot rather than
daemon-sharing. Happy to reconcile approaches if useful once #972 lands.

Changes

• src/session-cache.ts — disk-persisted status snapshot
  (loadStatusSnapshot/saveStatusSnapshot) keyed by corpus fingerprint
  • resolved query; atomic temp+rename write at 0600; schema-version
    guard against a payload-shape drift across binary upgrades; orphaned
    temp-file sweep extended to cover the new file.
• src/parser.ts — computeCorpusFingerprint, a stat-only pass over
  every discovered source (expanding Claude's directory-shaped sources
  to their actual .jsonl files first, since a directory's own mtime
  doesn't move when an existing file inside it is rewritten in place).
• src/main.ts — wires the status --format menubar-json path to check
  the snapshot before parsing/aggregating, and to persist a fresh one
  after a real recompute; a single shared options object now feeds both
  the cache key and the buildMenubarPayloadForRange call so they can't
  drift apart.
• tests/cli-status-menubar.test.ts — new coverage: snapshot-hit
  byte-identical output, debounce-defer during the settle window,
  reflect-after-settle.
• SPEC-perf-cache-fix.md — design write-up (root-cause investigation,
  why the debounce lives in the snapshot layer rather than
  reconcileFile, and why a first attempt at the latter was reverted).

Test plan

• npm test — full suite passes, including the new
  tests/cli-status-menubar.test.ts coverage.
• npx tsc --noEmit clean.
• Verified live against a real ~386MB session-cache corpus: cold call
  ~18-23s (unchanged — that's the underlying parse+aggregate cost, not
  addressed by this fix), settled repeat call ~2s, byte-identical
  payload.
• Manually installed via npm link and run against the real
  CodeBurnMenubar.app.

Checklist

• Code follows project conventions and coding standards
• Unit tests written and passing
• No new secrets, credentials, or API keys committed
• Release notes updated (docs/release-notes.md + README.md)
• npm test green